Tumorigenesis is generally charactenzed by multiple genetic alterations occurring in both dominant oncogenes and tumor suppressor genes. The p53 tumor suppressor gene is the most commonly mutated gene detected in human cancer. The wild-type p53 tumor suppressor gene product is believed to act as a safeguard against cancer whereas, mutant p53 fiinctions as an oncogene and promotes tumorigenesis. Mutant p53 may function in tumorigenesis by three different mechanisms: 1) By a loss of wild-type tumor suppressor function; 2) Inactivation of the wild-type p53 protein in a trans-dominant negative manner; and 3) by a gain of fiinction that promotes the tumorigenic potential of the cell. The long term goal of this research proposal is to further our understanding of the function of mutant p53 in promoting malignant transformation. Little is known about the "gain of fiinction" expressed by mutant p53 in promoting tumorigenicity. The gain of fiinction phenotype of mutant p53 is supported by the finding that the expression of exogenous mutant p53 in a cell line that is non-tumorigenic and devoid of endogenous p53 protein can transform these cells into a tumorigenic cell line. In addition, mutant p53 protein must be nuclear localized to enhance the tumorigenlc potential of the ceU, suggesting that mutant p53 may fiinction as a regulator of gene expression. Consistent with this hypothesis, mutant human p53 alleles, unlike the wild-type p53 protein, can selectively transactivate the human multi-drug resistance promoter in transient transfection assays using cells that are devoid of endogenous p53 protein. To advance our understanding of the mutant p53-gain-of-fiinction phenotype, we specifically propose to detail the molecular mechanisms by which mutant p53 fiinctions as a transactivator of gene expression by addressing the following questions: 1) What DNA sequences of the MDR promoter are required for mutant p53 trans tivation? 2) Is the endogenous MDR gene transactivated by mutant p53 in stably transfected cells? 3) What domains of mutant p53 are required for transactivation? 4) Does mdm-2, a p53 associated protein, mediate or regulate mutant p53 transactivation function? The proposed research will contribute to our general understanding of how mutant p53 may fiinction in cooperation with other factors, such as mdm-2, to promote tumorigenicity.